Ultra-High Strength 50SiMnCr Spring Flat Steel and Its Continuous Cooling Transformation Behavior

The development and application of ultra-high strength spring flat steel is of great significance in achieving automotive lightweight. Understanding the continuous cooling transformation behavior of this type of steel aids in developing its post-hot rolling cooling and quenching-tempering processes. On the basis of mechanical property testing， this study focuses on a newly designed ultra-high strength flat spring steel， 50SiMnCr， to investigate the effect of cooling rate on the microstructure and hardness of experimental steel by a combination of metallographic-hardness and thermal expansion method， thereby constructing continuous cooling transformation （CCT） curves. The results show that the strength and ductility of 50SiMnCr steel meet the requirements of the 2 000 MPa level， with phase transition point <italic>A</italic>_c1=721 ℃， <italic>A</italic>_c3=758 ℃， <italic>M</italic>_s=220 ℃ and <italic>M</italic>_f=103 ℃. Ferrite and pearlite transformation occurs when the cooling rate is 0.05 ℃/s-0.5 ℃/s. Bainite transformation takes place when the cooling rate is increased to 1 ℃/s. Martensite transformation happens when the cooling rate is above 2 ℃/s. The hardness of the this steel increases with increasing the cooling rate.